Isotopic tracers for the analysis of vegetation-derived organic matter in lignite-containing soils and sediments along a transect ranging from a forest soil to submerged lake sediment

Abstract

The mine soils and sediments in the Lusatian open-cast lignite mining district (Germany) contain substantial amounts of lignite in addition to recent organic matter derived from plant litter. Knowledge of organic matter (OM) types and their transformation in mine soils and sediments is essential for the evaluation of ecosystem functioning in this region. The aim of the study was to use the isotopic signature of OM to: (1) determine lignite C contribution and (2) elucidate the contribution of OM derived from plant litter to three physical fractions in order to deduce its degree of degradation in three contrasting environments. We used stable and radioactive C isotopes to quantify the contribution of lignite C and to characterize humification processes occurring under contrasting vegetation and redox conditions. The upper mineral soil/sediment, the litter layer and the vegetation of three plots were sampled along a transect covering a forest soil, a partially submerged sediment at the land–water interface characterized by fluctuating water levels, and a constantly submerged sediment. The soil/sediment samples were fractionated into a macro (>250 μm), a humus (material separated by flotation) and a sand (mainly mineral material) fraction to follow the transformation processes of plant and lignite. Radiocarbon analysis of the humus fraction showed a lignite C contribution ranging from 20% to 80% of total organic carbon (OC). The 14C activity was correlated with the δ 13C value ( r 2 = 0.95). Even if not precise, in this case the correlation was used to get an idea about the lignite contribution in coal mining-impacted freshwater sediment and soil. The 13C data showed that lignite C contribution increases with depth in every fraction of the forest and partially submerged plots. At the submerged plot, the humus and macro fraction were almost free of lignite C, suggesting high amounts of fresh plant material at this plot. This was confirmed by the highest contribution (up to 40%) of the macro fraction, which is mainly composed of fresh plant material, to the total OC content in this plot. In the first 5 cm of all plots, most OC was found in the humus fraction. Combined elemental and isotopic analysis of the three physical fractions indicated that high amounts of humified OM were present in the constantly submerged sediment, whereas most intensive degradation of OM was occurring in the partially submerged sediment at the land–water interface. Here, δ 13C values show that lignite C contributes to the macro fraction, which could suggest that it may have been incorporated into fresh plant material, thus being part of the active C cycle.